[OpenMP] Fix libomptarget build issue (#74067)

Libomptarget cannot be build because of the recent refactoring
introduced in patch 148dec9f :
[OpenMP][NFC] Separate Envar (environment variable) handling (#73994)

That patch moved handling of environment variables from libomptarget
library. That's why we don't need usage of "llvm::omp::target" namespace
if we handle environment variables.